Electric valve circuits



Aug. 9, 1938. B. D. BEDFORD 2,126,603

ELECTRIC VALVE CIRCUITS Original Filed Aug. 20, 1956 3 Sheets-Sheet 1 F ig2. 9 v c Inventor: Bumnice D. Bed-Ford Attorneg.

A g 9, B. D. BEDFORD 2,126,603

' .ELE'gTRIc VALVE CIRCUITS Original Filed Aug. 20, 1936 I5 Sheets-Sheet 2 I9 20 a; g A, 30 A, 30 30 HQ l I Pg slr h gJG KI- J Inventor:

His Attornek Aug. 9, 1938.

B. D. BEDFORD 2,126,603

ELECTRIC VALVE CIRCUITS firigin'al Filed Aug. 20, 1936 5 Sheets-Sheet 3 Burnice D. Bedflard by ofney.

Patented Aug. 9, 1938 PATENT OFFICE ELECTRIC- VALVE CIRCUITS Burnice D. Bedford, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application August 20,

1936, Serial No. 97,010

Renewed January 29, 1938 14 Claims.

My invention relates to'electric valve translating circuits and more particularly to control circuits for electric Valve translating apparatus.

In electric translating circuits including electric valve means of the type using ionizable mediums such as gases or vapors, there has been evidenced a decided need for apparatus to eliminate or reduce to a minimum the arc-back conditions to which these electric'valves may be subjected. It has been found that arc-backs may be eliminated or substantially reduced in number by preventing the establishment of high voltage gradients within the electric valve means during the commutation periods. It has also been found that the establishment of these high voltage gradients is in a large measure due to the increase in ion concentration or storage of ions within the electric valve means at the time the current is commutated between the valves.

-' It is an object of my invention to provide a new and improved control circuit for electric valve means of the type employing ionizable mediums.

It is another object of my invention to provide a new and improved electric valve circuit for preventing arc-back conditions within electric valves of the type employing ionizable mediums.

In accordance with the illustrated embodiments of my invention, I provide a control circuit for electric valves of the type employing ionizable mediums by which arc-back conditions of the electric valve means are substantially eliminated or materially reduced in number. In particular, I employ-a reactor, which may be of the saturable type, having a winding connected in series relation with the electric valve means for controlling the rate of change of current through the electric valve means at the end of the conducting periods and during the commutation periods. The reactor is also provided'with a control winding whichcontrols the magnetic condition of the reactor and hence efiects control of the inductance of the winding which is connected in series relation with the associated electric valve means. The reactor is arranged or designed to become substantially saturated at a relatively small percentage of the normal full load current which the series winding is designed to conduct. By controlling the energization of the control winding, I'pro-vide an arrangement whereby the reactor is saturated prior to and during the greater part of the conducting period so that the reactor offers a relatively small inductive reactance to the flow of current. In several modifications of. my invention I provide arrangements for energizing the control winding to effect saturation of the reactor by the control winding prior to the beginning of the normal conduction period,

in this way providing arrangements for introducing in the electric valve circuits only a relatively 5 small inductive reactance so that the power factor of the circuit is not materially reduced. The control winding eilects a substantial increase in the inductance of the series winding near or at the end of the conducting periods so that the 10 rate of change of current through the electric valve means is reduced below the rate of change of current, which for the particular electric valve means employed, establishes dangerously high voltage gradients within the electric valve means. 15 In other words, the rate of change of current is maintained below a predetermined value so that the positive ions associated with the ion plasma within the electric valve are allowed a sufficient time to diffuse within the electric valve means 20 without establishing high voltage gradients.

For a better understanding of my invention, reference may be had to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the 25 appended claims.

Fig. 1 of the accompanying drawings diagrammatically illustrates an embodiment of my invention applied to a bi-phase rectifier in which controlling reactors are connected in series relation with the electric valve means, and Figs. 2 and 3 represent certain operating characteristics of the circuit shown in Fig. 1. Fig. 4 diagram matically shows another embodiment of my invention as applied to an electric valve translating circuit for transmitting energy between a three phase alternating current circuit and a direct current load circuit, and Fig. 5 represents certain operating characteristics of the arrangement shown in Fig. 4. Fig. 6 diagrammatically represents another embodiment of my invention as applied to a quarter phase electric valve translating system, and Fig. 7 diagrammatically shows another embodiment of my invention as applied to an arrangement for translating energy between a three phase alternating current circuit and a direct current circuit.

Referring now to Fig. 1 of the accompanying drawings, my invention is diagrammatically shown as applied to an electric valve translating system for transmitting energy between an alternating current circuit I and a direct current circuit 2 through a transformer 3 and electric valve means 4 and 5 preferably of the type employing an ionizable medium such as a gas or a vapor. 55

The conductivity of the electric valve means 4 and 5 may be controlled by means of any conventional excitation or control circuit such as the excitation circuit 6 including a transformer I, a conventional phase shifting arrangement such as a rotary phase shifter 8, current limiting resistances 9 and any suitable source of biasing potential such as a battery Ifl.

In order to control the current through the electric valve means 4 and 5 at the end of the conduction period, I provide saturable reactors II and I2 each including a core member I3, a winding I4 connected in series relation with the associated electric valve means, and a control winding I5. The control windings I5 are energized in accordance with the potential appearing across the secondary winding of transformer 3 and are connected in series relation with an inductance I6 of relatively large magnitude. The saturable reactors II and I2 are designed so that the core members I3 becom'e saturated by a relatively small magnetomotive force as compared to the magnetomotive force incident to the flow of the normal full load current through the winding l4. In other words, the core members I3 of saturable reactors II and I2 become saturated at relatively small values of magnetomotive force and the windings I5 are provided with a small number of turns, whereas the windings I4 are provided with a relatively greater number of turns.

The operation of the electric valve translating circuit diagrammatically shown in Fig. 1 may be best explained by considering the operating characteristics illustrated in Figs. 2 and 3. As will be well understood by those skilled in the art, the electric valve means 4 and 5 will be rendered alternately conductive by the excitation circuit 6 so that each of these electric valve means conducts current for substantially 180 electrical degrees. The current conducted by electric valve means 4 may be represented by curve A of Fig. 2 and the current conducted by electric valve means 5 may be represented by the curve B of Fig. 2. By virtue of the presence of the large inductance IB connected in series relation with control windings I5 of saturable reactors II and I2, the magnetomotive force established in the core members I3 of these reactors may be made to attain the position represented by curve C of Fig. 2. Referring in particular to point a of curve A, which represents the begininng of a conducting period for electric valve means 4, it will be noted that since the current through control winding I5 is in a positive direction and of substantially maximum magnitude, control winding I5 will efiect substantial saturation of the core member I3 of reactor I I so that the inductive reactance offered to the flow of current through winding I4 will be relatively small and will permit the current to rise rapidly. It will be further noted that at the beginning of the conduction interval the current through winding I5 establishes a flux in the core member I3 which assists the flux established by the series winding I4. Curve 'E of Fig. 3 represents the magnetization curve of the reactors II and I2. At the beginning of the commutation period as represented by the point a of curve A, the saturable reactor II is substantially saturated and may be represented as operating on the portion of curve E lying beyond the point 11. On the other hand, it will be noted that during the interval bc of curve A, the rate of decay of current through the electric valve means 4 is substantially reduced so that near the end of the conducting period and during the commutating period the positive ions within the electric valve means are aiforded an opportunity to difiuse without establishing high voltage gradients within the electric valve means. This decrease in the rate of decay of current as represented by the portion of curve A between the points b and c is efl'ected by the increase in the inductance of the winding I 4. It will be noted that during this interval the magnetomotive force as represented by curve C has reversed in direction and attained practically maximum amplitude so that the core member I3 of reactor I I is not saturated during the interval bc and is operating in the vicinity of point e along the linear portion of the magnetization curve E. Curve D represents the voltage induced in windings I4.

A particular feature to be noted in connection with the embodiment of my invention diagrammatically shown in Fig. l is the characteristic of operation by virtue of which the reactors II and I2 offer a relatively small inductive reactance to the flow of current at the beginning of the conducting periods. As will be appreciated by those skilled in the art, in order to maintain a relatively high power factor operating condition it is desirable that the current begin to flow practically coincidentally with the application of voltage to the electric valve means. Since the reactors II and I2 are saturated by the control windings I5 prior to the beginning of the conducting periods, the current through the electric valve means, as represented by curves A and B of Fig. 2, may increase very rapidly. In addition, the apparatus possesses the desirable characteristic of automatically increasing the inductance of the windings I4 near the end of the conducting periods so that the current decreases at a slower rate to permit the positive ions within the electric valve means 4 and 5 to diffuse. While my invention as illustrated in Fig. 1 has been described in connection with an electric valve translating system for transmitting energy between a single alternating current circuit and a direct current circuit, it should be understood that in its broader aspects my invention may be applied to electric valve translating circuits generally for transmitting energy in either direction between single phase or polyphase alternating current circuits and direct current circuits, or between alternating current circuits of the same or different frequencies.

Fig. 4 of the accompanying drawings diagrammatically shows an embodiment of my invention as applied to an electric valve translating system for transmitting energy between an alternating current circuit I! and a direct current circuit I 8 through electric valve means I9-24, inelusive. Electric valve means I9-24 are preferably of the type employing an ionizable medium such as a gas or a vapor. The conductivity of electric valves I9-24 is controlled by means of an excitation circuit including a transformer 26 having a primary winding 2'! and secondary windings 2B. The excitation circuit 25 may be energized from any suitable source of alternating current, such as the alternating current circuit I1, through any conventional phase shifting arrangement such as the rotary phase shifting device 29. Current limiting resistances 30 are connected in series relation with control members of each of the electric valve means I944, and biasing potentials are impressed on the control members of these valves by any suitable biasing means such as batteries 3|.

Connected in series relation with the alternating current circuit I1 and the electric valve means I9, 22; 20, 23 and 2|, 24, I employ saturable reactors 32, 33 and 34 each including a core member 35, a winding 36 and a control. winding 31. The windings 36 are employed to control the rate of change of current through the electric valves near the end of the conducting periods, and the inductance of the windings 36 is controlled by the control windings 31. The core members 35 of the saturable reactors 32, 33 and 34 are designed so that substantial saturation is effected by a small percentage of the normal full load current which flows through the windings 36. In other words, the core members 35 are designed to saturate at relatively small values of magnetomotive force as compared with the magnetomotive force impressed on the core members when normal full load current is flowing through the windings 36. Control windings 31 are provided with smaller number of turns than the inductively associated series windings 36 and are capable of independently effecting substantial saturation of the core members 35 For the purpose of explaining the operation of the embodiment of my invention diagrammatically illustrated in Fig. 4, let it be assumed that the electric valve translating apparatus is operating to transmit energy from the alternating current circuit H to the direct current circuit l8. As will be well understood by those skilled in the art, the electric valve means Ill-24, inclusive, will be rendered alternately conductive and non-conductive to effect full wave rectification of the alternating current. It will also be noted that the pairs of oppositely disposed electric valves, such as valves l9 and 22, will be rendered conductive substantially 180 electrical degrees out of phase and will not be conductive during the same intervals Furthermore, it will be noted that each electric valve conducts current during two electrical degree intervals during each cycle of alternating potential, and that during each of these intervals any one electric valve will conduct only 60 electrical degrees with any one other electric valve. For example, electric valves 20 and 24 will conduct current for substantially 60 electrical degrees and electric valves 20 and 22 will conduct current during the next succeeding 60 electrical degree interval, the current having been commutated at the end of the first 60 electrical degree interval from the electric valve 24 to the electric valve 22.

Considering the operatingcharacteristics of the electric valve translating system as shown in Fig. 5, curve F may be employed to represent the current which is conducted through winding 36 of saturable reactor 32, and curve G may be employed to represent the current which is conducted through winding 36 of saturable reactor 34 and control winding 31 of saturable reactor 32. The phase relationship of these currents will be apparent in view of the above discussed principles or operation. During the interval ;fg, there will be no current conducted through winding 36 of saturable reactor 32, but the current through control winding 31 of saturable reactor 32 will be of substantially full load value and the magnetomotive force of this winding will be impressed on core member 35 of reactor 32. The magnetomotive force impressed on the core member '35 of saturable reactor 32 may be represented by curve H of Fig. 5. Since the series winding 36 of reactor 34 is connected in series relation with control winding 31 of reactor 32, the magnetic condition of core member 35 of reactor 32 is controlled in accordance with the current of this phase in the polyphase system. By virtue of this current flowing through the control winding 31 of reactor 32, the core member 35 is substantially saturated at the time represented by the point g so that when electric valve 23 is rendered conductive to commutate current from electric valve 22 the inductive reactance offered by series winding 36 will be relatively small so that the current may increase rapidly and will not materially influence the power factor conditions imposed on the alternating current circuit ll. During the interval gh electric valves .23 and 2| will be conductive to transmit current from the alternating current circuit l I to the direct current circuit l8 through control winding 31 of reactor 34, series winding 36 of reactor 33, one phase of the circuit I1, series winding 36 of reactor 32, and control winding 31 of reactor 33. During the interval h-i, current will be supplied to the direct current circuit l8 from the alternating current circuit l1 through electric valves l9 and 23, the current having been commutated from electric valve 2| to electric valve I!) at the time corresponding to point It. It will be noted that during the interval h-i the direction of current transfer through the control winding 31 of reactor 32 has been reversed relative to the direction of current through this winding during the interval f-Q. effecting thereby a reduction in the net magnetomotive force impressed on core member 35. At the time corresponding to point 1 current will be commutated from electric valve 23 to electric valve 24. During the commutation period as represented by the portion of the curve F lying within the interval 12-4, the current through the electric valve means 23 decays at a relatively slow rate by virtue of the increased inductance effected by the oppositely directed magnetomotive forces impressed on the core member 35 of reactor 32 by means of control winding 31. duced in windings 31 of saturable reactor 32. In a similar manner, the saturable reactors 33 and 34 will control the rate of change of current to the electric valve means Iii-24 at the end of the conducting periods or during the commutating periods to prevent the establishment of high voltage gradients within the electric valve means by allowing sufficient time for the positive ions to diffuse.

A particular feature to be considered in con nection with the arrangement of Fig. 4 is that I provide a means for efiecting substantial saturation of the saturable reactors 32, 33 and 34 at the beginning of the conducting periods so that the current increases very rapidly, and it should be further understood that I provide an arrangement whereby the saturation of the reactors is automatically reduced prior to the commutation periods so that the rate of change of current through the electric valve means is decreased below a predetermined value to allow sufiicient time for the diffusion of positive ions within the electric valve means and to prevent the establishment of high voltage gradients within the electric valve means. It should be further understood that this control of the magnetic condition of the saturable reactors 32, 33 and 34 is effected by employing the currents of associated phases in a polyp-base alternating current system. In the arrangement shown in Fig. 4, the same effect may be obtained Curve J represents the voltage inby reversing the direction of the control windings 31 of saturable reactors. 32, 33 and 34 and by adopting the opposite phase rotation.

Fig. 6 of the accompanying drawings represents another embodiment of my invention as applied to an electric valve translating system for transmitting energy between a three phase alternating current circuit 38 and a direct current circuit 39 through electric valve means 49, 4|, 42 and 43 preferably of the type employing ionizable mediums such as gases or vapors. Interposed between the three phase alternating current circuit 38 and the electric valve means 46 to 43, inclusive, I employ a transformer 44 of the type for effecting three phase-quarter phase voltage transformation having a primary winding 45 and secondary windings 46, 41, 48 and 49. Connected in series relation with the secondary windings of the transformer 44 and the electric valve means 46 to 43, I provide saturable reactors 50 and 5| having windings 52, 53, 54 and 55, respectively. Windings 52 and 53 are connected in series relation with secondary windings 46 and 43, respectively, of transformer 44, and windings 54 and 55 are connected in series relation with secondary windings 41 and 49, respectively, of transformer 64. Reactors 56 and 5| are also provided with control windings 56, 51 and 58, 59, respectively, which control the magnetic condition of the core members of reactors 50 and 5 I, respectively, and which thereby effect control of the inductance of series windings 52 to 55, inclusive. Control windings 55 and 51 are energized in accordance with the current of secondary windings 41 and 45L respectively, of transformer 44, and control windings 58 and 59 are energized in accordance with the current of secondary windings 46 and 46 of transformer 44. The control windings 56 to 59, inclusive, are provided with relatively smaller number of turns than the series windings 52 to 55, inclussive, but are designed to effect substantial saturation and desaturation of the respective core members during the predetermined desired starting and commutating intervals.

The operation of the electric valve translating system diagrammatically shown in Fig. 6 is substantially the same as that described above in connection with the operation of the arrangement shown in Fig. 4. As will be well understood by those skilled in the art, electric valves 4643, inclusive, will be rendered conductive in a predetermined order to transmit current from the three phase alternating current circuit 38 to the direct current circuit 39. Control windings 56 to 59, inclusive, will effect substantial saturation of the core members of reactors 56 and 5| so that the inductive reactance offered to the flow of current in series windings 52 to 54 will be relatively small at the beginning of the conduction periods. Furthermore, control windings 56 to 59, inclusive, will control the magnetic condition of the core members of reactors 56 and 5| to effect a substantial increase in the inductance of series windings 52 to 55, inclusive, near the end of the conducting periods and during the commutating periods so that the rate of change of current is maintained within a predetermined range.-

Fig. 7 of the accompanying drawings diagrammatically shows another embodiment of my invention as applied to an electric valve translating system for transmitting energy between an alternating current circuit 66 and a direct current circuit 6| through electric valve means 62-61, inelusive, and through a transformer 68. The control or excitation circuits for electric valves 62 to 61 are not shown but it will be understood that any of the conventional arrangements shown in the art may be applied to control the conductivity of these electric valves. Interposed between the secondary windings of transformer 66 and the electric valves 62 to 61, inclusive, I provide saturable reactors 69, 16 and H having core members 12, 13 and 14, respectively, and each having a pair of similarly disposed series windings 15, 16 and each having a control winding 11. Windings 15 of reactors 69, 16 and 1| are connected in series relation with electric valves 62, 63 and 64 respectively, and windings 16 of these reactors are connected in series relation with electric valves 65, 66 and 61. The control windings 11 are energized in accordance with the current of the direct current circuit 6| and are connected in series relation with the electric valves 65, 66 and 61. The control windings 11 establish in the core members 12, 13 and 14 a magnetomotive force which opposes the magnetomotive forces established by the series windings 15 and 16.

The electric valve translating system of the type shown in Fig. 7, and the associated reactors 69, 16 and 1| may be employed in those applications where it is not objectionable to incur a slight decrease in the power factor of the alternating current load imposed on the alternating current supply circuit. Since the control windlugs 11 at all times impress on the core members 12, 13 and 14 of saturable reactors 69, 16 and 1| magnetomotive forces tending to oppose the magnetomotive forces established by the series windings 15 and 16, these control windings will tend to effect an increase in the inductance of the series windings 15 and 16 at both the beginning and the end of the conducting periods. Since the rate of change of current near the end of the conducting period will be decreased, the arrangement of Fig, 7 will also be effective to prevent the establishment of high voltage gradients within the electric valve means and will be effective to prevent arc-back failures or reduce to a minimum the number of arc-back failures.

While I have shown and described my invention as applied to a particular system of connections and as embodying various devices diagrammatically shown, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention.

and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination, an electric translating system including an electric valve means connected therein and arranged to conduct current during predetermined intervals, and means for controlling the current conducted by said electric valve means at the end of the conducting intervals comprising a saturable reactor having a winding connected in series relation with said electric valve means and a winding for increasing the inductance of said first mentioned winding at the end of the conducting intervals.

2. In combination, an electric translating system including electric valvemeans connected therein and arranged to conduct current during predetermined intervals, and means connected in series relation with the electric valve means having a. relatively small inductance at the beginning of said conducting intervals and arranged to effect a substantial increase in the inductance at the end of the conducting intervals to control the 1 efiect substantial saturation of said core member at the beginning of each conduction period and to decrease the magnetization of said reactor to effect an increase in the inductance of said first mentioned winding at the end of each conduction period. 1

4. In combination, an electric translating system comprising a plurality of electric circuits, a plurality of electric valve means connected in said circuits and arranged to conduct current during predetermined intervals, and means for controlling the rate of change of current through said electric valve means at theend of the conducting intervals comprising a plurality of saturable reactors each associated with a difierent one of said electric circuits and each comprising a winding connected in series relation with said electric valve means and each comprising another winding energized in accordance with the current of another of said electric circuits to increase the inductance of said first mentioned winding at the end of the conducting intervals.

5. In combination, an electric translating system including a plurality of electric valve means connected therein and arranged to conduct current during predetermined intervals and in a predetermined order, and means for controlling the current conducted by said electric valve means when the current is commutated from one of said electric valve means to another of said electric valve means comprising a plurality of saturable reactors each having a winding connected in series relation with the associated electric valve means .and each having another winding energized in accordance with the current conducted by another of said electric valve means to control periodically the inductance of said first mentioned associated winding.

6. In combination, an electric translating system comprising a plurality of alternating current circuits, a plurality of electric valve means connected in said circuits and arranged to conduct current during predetermined intervals, and means for controlling the current through said electric valve means at the end of the conducting intervals comprising a plurality of saturable reactors each associated with a different one of said electric valve means and each having a winding connected in series relation with the associated electric valve means and a winding inductively associated with said first mentioned winding and energized in accordance with the current of another of said electric valve means to increase periodically the inductance of said first mentioned associated winding.

'7. In combination, an electric translating system comprising a plurality of electric conductors, said conductors being arranged to conduct polyphase alternating current, a plurality of electric valve means each associated with a different one of said conductors and arranged to conduct current during predetermined intervals, a plurality of saturable reactors each associated with a different one of said electric valve means and each comprising a winding connected in series relation with the associated electric valve means and each comprising a winding inductively associatedwith' saidfirst mentioned winding for increasing the inductance of said first mentioned winding to effect a decrease'in the rate of change of current at the end of said conducting intervals.

8. In combination, a polyphase alternating current circuit, a'direct current circuit, an electric translating circuit interposed between said circuits andincludinga plurality of electric valve means arranged to conduct current during predetermined intervals and in a predetermined order, and a pluralityof saturable reactors each associated with a difierent one of said electric valve means and each comprising a winding connected in series relation with the associated elec tric'valve means andeach comprising a second winding energized in accordance with the current of another phase of said alternating current circuit to increase the inductance of said first men tioned winding when current is commutated from the associated electric valve means.

9. In combination, an electric translating system including a polyphase alternating current circuit, a plurality of electric valve means connected in said alternating current circuit and arranged to conduct current during predetermined intervals and in a predetermined order, and means associated with each of said electric valve means comprising a plurality of saturable reactors each including a core member, a winding connected in series relation with the associated electric valve means and a second winding energized in accordance with the current of a different phase of said polyphase circuit for increasing the inductance of said first mentioned winding to control the rate of change of current through the associated electric valve means when the current is commutated therefrom.

10. In combination, an electric translating system including an electric valve means connected therein and arranged to conduct current during predetermined intervals, and means for control ling the rate of change of current through said electric valve means at the end of the conducting intervals comprising a saturable reactor including a core member, a winding connected in series relation with said electric valve means and a control winding for controlling the magnetic condition of said core member, said control winding being energized to establish in said core member at the beginning of each conducting interval a flux in the same direction as that establish-ed by the first mentioned winding and tending to establish in said core member at the end of each conducting interval a flux which opposes the flux established by said first mentioned winding.

11. In combination, a polyphase alternating current circuit, a direct current circuit, and electric translating apparatus interposed between said circuits including a plurality of electric valve means and a plurality of saturable reactors each having a core member, a winding connected in series relation with a predetermined different one of said electric valve means for controlling the rate of change of current at the end of the conducting intervals and a control winding energized in accordance with the current conducted by a different electric valve means for establishing a flux in said core member at the beginning of the conducting intervals in the same direction as the flux established by said first mentioned winding and tending to establish a flux in the opposite direction. at the end of the conducting intervals.

12. In combination, a polyphase alternating current circuit, a direct current circuit, and electric translating apparatus interposed between said circuits including a plurality of pairs of electric valve means serially connected relative to said direct current circuit and a plurality of saturable reactors interposed between said alternating current circuit and said electric valve means, each of said reactors including a winding connected in series relation with a predetermined pair of serially-connected electric valve means and a control winding energized in accordance with the current conducted by a different pair of seriallyconnected electric valve means for increasing the inductance of said first mentioned winding at the end of the conduction intervals.

13. In combination, an alternating current circuit, a direct current circuit, electric translating apparatus interposed between said circuits including a plurality of electric valve means, and means comprising a plurality of saturable reactors each associated with a different electric valve means and each including a core member, a winding connected in series relation with the associated electric valve means and a winding for establishing a unidirectional fiuX in said core member to decrease the rate of change of current through said associated electric valve means at the end of the conducting intervals.

14. In combination, a three phase alternating current circuit, a direct current circuit, and electric translating apparatus interposed between said circuits including a transformer for obtaining three phase-quarter phase voltage transformation, a plurality of electric valve means each associated with a different secondary phase winding of said transformer and two saturable reactors interposed between said transformer and said electric valve means, each of said reactors including twowindings each connected in series relation with a predetermined different one of said electric valve means and two control windings each energized in accordance with the current conducted by a different electric valve means for increasing theinductance of said first mentioned windings at the end of the conduction intervals of the associated electric valve means.

BURNICE D. BEDFORD. 

